Electrostatic atomization of conductive paints

ABSTRACT

Conductive paints are electrostatically sprayed by electrically isolating the electrostatic charge imposed on the atomizing head from the paint supply tank to prevent dissipation of the charge through the conductive paint which is supplied.

United States Patent Rokadia 1 July 8, 1975 [54] ELECTROSTATICATOMIZATION OF 636,234 11/1899 Baker 204/220 X 2,414,741 1/1947 HubbardCONDUCTIVE PAINTS 3,122,320 2/1964 Beck et al 117/93 X [75] Inventor:Abdullah M. Rokadia, Schaumberg,

lll.

Prima Examiner-Robert S. Ward Jr. 7 D t 11. W I 3] Asslgnee 880 0 IncDes Flames l Attorney, Agent, or FirmDressler, Goldsmith, [22] Filed:Oct. 4, 1973 Clement & Gordon, Ltd. [21] Appl. No.: 403,364

[52] US. Cl 239/3; 239/15; 1|7/93.4 R; [57] ABSTRACT 1 18/629 [51] Int.Cl B0511 5/02 Conductive paints are electrostatically sprayed by 1 FieldSearch 117/914 electrically isolating the electrostatic charge imposed220 on the atomizing head from the paint supply tank to preventdissipation of the charge through the conduc- [56] References Cited tivepaint which is supplied.

UNITED STATES PATENTS 8/1897 Kellner 204/220 X 1 Claim, 1 Drawing Figure1 ELECTROSTATIC ATOMIZATION OF CONDUCTIVE PAINTS The present inventionrelates to the electrostatic spraying of conductive paints usingequipment in which the electrostatic charge is employed to assist in theatomization of the paint. More particularly, this invention relates tothe electrical isolation of the electrostatic charge from the paintsupply tank in order that the charge not dissipate back through thepaint supply to create a source of considerable hazard, and interferewith the electrostatic atomization by dissipating the charge on theatomizing head.

In the conventional electrostatic spray system, involving electrostaticatomization, the paint is pumped through a supply tube to a chargedrotating disc, or other appropriate spray head, and the charge passesthrough the paint on the disc to assist its atomization. However,whenever the paint is unduly conductive, the charge shorts out throughthe conductive paint supply, hampering electrostatic atomization andcreating an obvious hazard. The conventional practice, therefore, is tolimit the conductivity of the paint which is used. Another alternativeis to insulate the entire system, including the paint supply tank. Thisis not only expen sive, but the system cannot be safely approached, andthere is a risk of accidental electrical shock which could be seriousand which cannot be eliminated. Of

course, one can use electrostatic spray equipment which does not involveelectrostatic atomization, but these are sometimes less satisfactory andthis suggestion does not help one who already possesses equipment forelectrostatic spray and simply wishes to apply a more conductive paint,e.g., an aluminum flakepigmented paint, or an aqueous paint which may bein solution or in emulsion.

The present invention allows the electrostatic spraying of electricallyconductive materials using electrostatic atomization by effectivelyisolating the electrostatic charge so that it is not conducted back tothe paint supply tank.

In one aspect of this invention, isolation of the electrical charge fromthe paint supply tank is achieved by including in the paint supplysystem a section in which the column of paint which is supplied isunconfined, e.g., in a free fall.

Remarkably, it has been found that conductive paint which will easilycarry the electrostatic charge when confined in a tube will not carrythe same charge for any distance when it is unconfined. Thus, a 50,000volt atomizing charge has been found to be conducted in a free fallingcolumn of conductive aqueous paint only about 2 feet. Since the systemtested involved a free fall of 5 feet, no charge could be detected onthe structure at the top of the column.

It will be appreciated that the distance which the charge can negotiatethrough the unconfined paint col umn will vary with such factors as thevelocity of the paint column, the conductivity of the paint, and thelevel of charge. The remarkable point is that, regardless of thevariation, the distance is limited, and normally does not exceed about 4feet. As long as this distance is exceeded, the charge will notdissipate, and the paint supply will remain uncharged and safe tohandle.

It should be kept in mind that the atomizing charge involves highvoltage and minimal amperage, and this factor is probably important tothe inability of the charge to be conducted through the unconfinedcolumn of paint.

The unconfined column of paint is fed to an electrically isolated feed.This isolated feed is fed by the unconfined column of paint through anintervening air space to isolate the feed from the paint supply tank.Thus, in the practice of the method of this invention, paint is suppliedto the electrostatically charged atomizing head from an electricallyisolated paint reservoir which is supplied with an unconfined column ofpaint through an intervening air space.

To further increase the security of operation, the paint being suppliedcan be supplied in spurts providing separate discrete paint portionswhich move to the isolated paint reservoir through the intervening airspace. Thus, in one form of the invention, the electrically insulatingcharacter of the unconfined paint is assisted by having the unconfinedpaint in the form of physically separated discrete portions or slugs.

Thus, the conductive paint is supplied through a tube and means areprovided to interrupt the flow on and off to divide the paint at thefree end of the tube into spaced apart drops or spurts which passthrough the intervening air space in the form of separated slugs tomaintain the desired pool of paint in the feed reservoir.

While the use of separated portions of paint increases the security ofthe system, it adds considerably to the cost of the system. The use of acontinuous column of unconfined paint is particularly valuable sinceconstruction costs are minimized and convenience of operation isenhanced.

The number and size of the separated slugs which are employed to providethe desired supplementary obstruction to the flow of electricity fromthe atomizing head back through the paint supply column will depend uponthe conductivity of the paint as well as the level of charge to bemaintained.

This invention is particularly applicable to highly conductive paints,such as aqueous paints based on acidic resins which are dispersed orstabilized with ammonia or an amine.

The conductivity of the aqueous paints to which this invention isparticularly applicable can vary considerably and is dependent on manyfactors. Thus, the amine concentration and the pH are important factorswhich will vary the conductivity and, correspondingly, the extent towhich the features of this invention must be used to provide effectiveelectrical isolation.

The spaced apart slugs of paint which may be supplied in accordance withthis invention may be produced in any desired manner, and this,therefore, is not a primary feature of the invention. An illustrativemethod of producing them would involve the use of a timer which opensand closes a valve to intermittently open and close the paint supplyline. A diagram pump will provide the same interrupted flow, but this isless preferred because of difficulty at the check valves.

The present invention is applicable to any electrostatic atomizingsystem which the conductivity of the paint may cause the charge appliedto the atomizing head to be conducted back through the paint supply lineto the paint supply tank. Illustrative of the kind of atomizing headwhich is employed in this invention is electrostatic discs or bells,these being, per se, well known in the art. However, the paint can beflowed to any charged surface or needle point so that the charge willpass through the body of the paint on its way to a grounded target and,as an incident to such passage, the paint will become atomized.Similarly, the spray function can be only partly electrostatic. In anyevent, the paint is directly charged so that the charge has theopportunity to short out through the paint supply, and this raises theproblem of the invention.

To particularly illustrate the use of this invention, a waterbased latexpaint may be sprayed electrostatically using an atomizing bell spinningat 1800 RPM and having a l00,000 volt charge thereon to maintain avoltage gradient of about l0,000 volts per inch with respect to agrounded target placed inches from the atomizing bell. This paint issupplied to an isolated reservoir through an air space 6 feet high. Thepaint is supplied through a steel tube one-fourth inch inside diameterat about 1250 grams per minute. Using a voltmeter reading up to 30,000volts, the voltage in the reservoir was over 30,000 volts. No voltagecould be detected at the supply tube above the reservoir even though theatmosphere was damp so that the charged area or field around thereservoir extended out 3 to 4 feet from the reservoir. The area ofcharge will vary with the charge used, and the humidity and the columnlength needed will vary with the paint conductivity and the paintvelocity.

The invention will be described more fully in connection with theaccompanying drawing in which the single figure is a diagrammatic viewshowing the use of an isolated feed to an electrostatic atomizing discwith unconfined paint being supplied to insulate the grounded feedsupply structure.

Referring more particularly to the drawing, paint supply tank 10supplies paint through pump 11 to a supply tube 12 which may beconductive or nonconductive as desired.

Located along the length of supply tube 12 is an air operated on-offvalve 13 which is conveniently positioned near the free end of the tubeas shown. Valve 13 is supplied with operating air through line 14.

Timer 16 operates a three-way solenoid valve 15. In the energizedposition with the timer on the on cycle, air under pressure shown byArrow A passes through line l4 to operate valve 13 to open the line 12.When the timer is in its off" position, solenoid valve is deenergizedand air in line 14 bleeds out through opening 17, and this causes valve13 to close, shutting off flow in line 12.

The paint [8 in the reservoir 10 flows through pump 1 l and forms acontinuous confined column in line 12. This column of paint is releasedand falls freely and unconfined into paint receiver 30. If the valve 13is operated, then the unconfined column of paint shown at 32 is brokeninto separated slugs or droplets.

Tube 12 terminates above an electrically isolated paint receiver 30having an inclined receiving surface 31 for the purpose of minimizingspattering as the unconfined column of paint 32 delivered from tube [2falls through the intervening air space to strike upon and then flowdown the inclined receiving surface 31 to merge into and become part ofpool 33.

From the pool 33 the paint flows by force of gravity through tube 34 theatomizing disc or bell 35 which is charged through terminal 36.

A gravity feed to the disc 35 is shown and is preferred, but an airoperated pump can be interposed in the line 34 to add flexibility in theprocess, or to increase the flow rate.

Normally, the paint will simply fall vertically through the interveningair space, as shown, but it may be propelled horizontally if desired.

The invention is defined in the claims which follow.

I claim:

1. A method for electrostatically spraying an electrically conductiveaqueous paint comprising supplying an unconfined continuous column ofsaid paint through an intervening air space to an electrically isolatedpaint reservoir, and then conveying said aqueous paint from saidelectrically isolated reservoir as a confined column in a paint supplyline to an electrostatically charged atomizing device so that saidelectrostatic charge is conducted back through said paint to theelectrically isolated paint reservoir to create a charged field aroundsaid reservoir, the paint providing said unconfined continuous columnsupplied to said reservoir being released to provide said unconfinedcolumn from a point at least about 2 feet above said reservoir.

1. A method for electrostatically spraying an electrically conductiveaqueous paint comprising supplying an unconfined continuous column ofsaid paint through an intervening air space to an electrically isolatedpaint reservoir, and then conveying said aqueous paint from saidelectrically isolated reservoir as a confined column in a paint supplyline to an electrostatically charged atomizing device so that saidelectrostatic charge is conducted back through said paint to theelectrically isolated paint reservoir to create a charged field aroundsaid reservoir, the paint providing said unconfined continuous columnsupplied to said reservoir being released to provIde said unconfinedcolumn from a point at least about 2 feet above said reservoir.